As wheely says, welding, brazing or soldering are the only options, rad sealers are a bodge, and most will start to leak when you put corrosion inhibitor or antifreeze in the system
It just depends on the type of rad construction as to what type of repair is needed, and how bad the damage is, as some it will be cheaper to get a recon rad rather than trying to repair it, especially if the damage is deep in the cooling matrix
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